Thread grinding mechanism and method



Patented Feb. 9, 1954 :1

THREAD GRINDING MECHANISM AND METHOD Charles A. Reimschissel,

signor to Landis Mach boro,

Waynesboro, Pa., as-

ine Company, Waynes- Pa., a corporation of Pennsylvania Application July 14, 1952, Serial No. 298,717

4 Claims. 7 (Cl. 51288) This invention relates to abrasive grinding Wheels for the production of screw threads in grinding machines, particularly centerless grinding machines and to methods of thread grinding.

In prior machines of this type, the abrasive grinding wheels as shown for example in Patent No. 2,624,159, dated January 6, 1953, comprise two chief grinding sections; a rough-grinding section for gradually generating the approximate thread profile desired and a finish-grinding portion having annular ridges of equal diameter for finishing the thread accurately to the desired size and contour. To increase the accuracy of the wheel it has been found desirable to add a plain cylindrical grinding section in advance of the roughgrinding section. This section reduces the work blank to the required diameter and prevents grinding the external diameter of the work during the subsequent thread grinding operation.

However, it has been determined that the action of a centerless grinding machine, when a plain cylinder is being ground, is such that the axial movement of the work piece does not bear the necessary exact relation to the rotation of the work piece. Consequently, when a thread grinding operation is preceded by a plain cylindrical grinding operation on the sam grinding wheel, the work piece arrives at the thread grinding ridges with an inaccurate rate of axial movement. The average error in axial feed of the work has been found to be approximately 3%. This is greatly in excess of the permissible variation in the lead of a screw thread. In the prior machines undue stress is imposed on the first roughing or thread generating ridges which are employed to correct this inaccuracy. As a result the ridges wear at an accelerated rate and maintenance and replacement costs may become excessive.

In accordance with the present invention the initial inaccuracy of the movement of the work piece is eliminated and th resulting excessive stresses in the succeeding forming ridges prevented by the provision of a novel preliminary control section at the entrance side of the thread grinding mechanism. The control section comprises a relatively short rough-grinding section of annular ridges in advance of the plain grinding section. These preliminary grinding ridges, which do not approach the finish-grinding ridges in size, nor necessarily in shape, provide a nut action which, in cooperation with the leading effect of the correctly inclined control or regulating wheel and the correctly inclined work rest blade, imparts a precisely accurate axial movement or lead to the work piece. These initial ridges also tend to seize the work blanks and start their axial movement more quickly, thus materially increasing production rates with a corresponding decrease in the unit cost.

It is, accordingly, the primary purposeand object of the present invention to provide improved thread grinding apparatus and methods which overcome these deficiencies of the prior art and which produce a superior product at reduced cost.

It is a further object to provide improved thread grinding mechanisms and methods in which provision is made for controlling with increased accuracy the movement of the work piece through the machine to reduce stresses imposed by the uncontrolled variation in the movement of the work piece common in prior machines.

It is also an object of the invention to provide a novel thread grinding wheel having means for grinding the external diameter of the work piece before the thread is completed so that grinding of the external diameter and grinding of the thread profile do not take place simultaneously.

It is a further object to provide grinding wheels having means to establish axial movement of the work piece at a predetermined rate before the external diameter is ground.

Another object of the invention is to provide a thread grinding wheel having a group of ridges at the entrance side for partial formation of the screw thread, followed successively by a plain cylindrical grinding section, a roughing threadgrinding section and a finishing thread-grinding section. v

A further object of the invention is to provide a thread grinding wheel,.for a centerless thread grinding machine, having a plain cylindrical section for grinding the external diameter. of a work piece, preceded by a group of annular ridges which provide suflicient nut action between work piece and grinding wheel to cause the axial movement of the work to be precis and unvarying during theoperation of the plain cylindrical section.

Other objects and advantages of th invention will be, apparent from the following description and the accompanying drawings, in which:

Figure l is a, diagrammatic representation of the principal elements of a centerless thread grinder;

Figure 2 is a fragmentary sectional view, on an enlarged, scale, taken substantially on line 22 of Figure 1; V V

Figure 3 is a fragmentary sectional view similar to a portion of Figure 2, showing a modification of the grinding wheel of that figure; and

Figure 4 is another fragmentary sectional view similar to a portion of Figure 2, showing a further modification of the grinding wheel of that figure.

The general arrangement of the major components of the apparatus of Figure 1, may take the form shown in the aforesaid Patent No. 2,624,159. An abrasive grinding wheel is shown in position to perform abrading operations upon a work piec 22 which is supported by a work rest blade 24 in the grinding throat between the grinding wheel 29 and a control wheel 26. The latter serves, in well known manner, to con trol the speed of rotation and rate of axial travel of work piece 22 as well as to urge it against the periphery of grinding wheel 20.

In grinding machines of this type the thread grinding wheel is always rotated in a clockwise direction as seen in Figure 1 so as to urge the work blank downwardly against the work rest blade. In ordinary work the rotation of the control wheel 26 in the clockwise direction as seen in Figure 1 results in satisfactory performance. However it has been found when a large amount of metal is to be removed from the work blank, as when grinding threads of coarse pitch, rotation of the control wheel in the opposite direction so that its surface moves downwardly toward the work rest is preferable and in some cases necessary.

A crush-dressing roller 28 is also shown in operative relation to grinding wheel 29, it being understood that this roller is not actually in operation while a work piece is being ground. The function of roller 28 is that of applying the desired contour to the peripheral face of grinding wheel 29. In the case of the present invention this contour comprises annular grooves for grinding screw threads upon the work. The hardened steel roller 28 may be associated with any well known mechanism capable of performing the operation of impressing the contour formed on its periphery into the softer abrasive material of grinding wheel 20. Such a mechanism, for example, is shown and described in Patent 2,601,290.

As shown in Figure 2, grinding wheel 26 is provided with a series of annular roughing ridges 30, extending from the point 32 to the point 34, increasing gradually in height toward the latter point. These ridges and the intervening grooves may be formed as described in the aforesaid Patent No. 2,624,159, but according to the present invention, their form is not limited to that described in the said application but may be any profile, beginning in fragmentary form at point 32 and progressively assuming a form approximately that of the final thread contour desired.

From point 34 to point 36 there is formed on the periphery of grinding wheel 20 a series of annular ridges 38 which are accurately complementary to the contour of the finished thread and which, in the operation of the machine, finish-grind the work piece after the preliminary grinding operation performed by the ridges 30. A cylindrical portion 40 may be provided between point 36 and the exit side 42 of the grinding wheel 20 to provide a guide for the work piece after it has passed the thread forming ridges. This portion 40, while advantageous in preventing scarring of the thread as the work leaves the machine, is not essential to the invention.

A cylindrical grinding portion 44 is provided on the grinding wheel 20, extending from point 32 toward the right as seen in Figure 2. Portion "44 is employed for the purpose of grinding the external diameter of the work blank and hence the major diameter of the screw thread accurately to the desired size, so that no grinding is done on the major diameter during the formation of the thread by ridges 30 and 38. Accordingly the diameter of the portion 44 is greater than that of the root surfaces of the thread grinding sections of the wheel.

To the right of cylindrical portion 44 is formed a group of roughing thread-grinding ridges 49 which extend to, and gradually decrease in diameter toward, the work entrance side 48 of the grinding wheel. The crest surfaces 58 of the ridges 46 are all equal in width and this width may approximate the crest width of the finished thread. The root surfaces 52 of the grooves between the ridges 46, however, are all of the same diameter and this diameter should be slightly less than that of the cylindrical grinding section 44 so that no grinding is done on the external surface of the work blank. Thus, the ridges 49 decrease in depth as well as in overall diameter toward the work entrance side 48 of the grinding wheel. The crests 59 of ridges 46 are spaced apart laterally a distance substantially equal to the pitch of the thread to be ground. As a result, the width of the root surfaces between ridges it increases toward the work entrance side of the grinding wheel 29. The maximum height of ridges 45, that is, the height of the ridge 56 next adjacent cylindrical grinding portion i is preferably about one third of the depth of the finished thread, although this dimension is not critical.

At the entrance side of the grinding wheel there is preferably provided a sharply tapered grinding portion 5 l This grinding portion is particularly useful in grinding work pieces which have not previously been chamfered on their leading ends or which are otherwise irregular. The ends of such work pieces are ground by the inclined section ill to a diameter suitable for passing the work piece into the grinding throat in contact with the grinding wheel surface.

In operation of the machine, a work blank 22 is introduced into the throat between grinding wheel 28 and control wheel 29 at the work entrance side 48. The control wheel 26, having its axis properly inclined, controls the speed of rotation of work piece 22 while the ridges 4'6 generate a helical groove in the work piece. The proper rate of axial advance of the work piece is assured by the action of the control wheel in cooperation with the nut action resulting from the engagement of ridges 46 with the work piece.

As stated above, the initial ridges 46, in addition to establishing the correct rate of axial advance of the work piece, also tend to seize the work blanks and start their axial movement more quickly than a plain cylindrical grinding surface because the separate crests of the thread ridges have much less metal to remove than a plain cylindrical surface. Also for this reason the work will start between the wheels with much less effort especially when the major diameter of the work blank is over size.

In the axial advance of the work piece, the plain cylindrical section 34 of the grinding wheel 20 is next encountered where only the external diameter of the work piece is ground. Here the proper rate of axial travel of the work piece 22 is maintained by making the width of the plain grinding portion 54 less than the length of the work piece so that the partially formed thread is constantly engaged either by ridges 46 or ridges 39. After passing the cylindrical grinding section 44, the work piece 22 has its thread gradually deepened and enlarged by the ridges un til the point 34 is reached. Thereafter the ridges 38 bring the screw thread accurately to finished size and contour.

Figure 3 shows a modification of the contour of the preliminary roughing ridges. Here the ridges 66a have originally been formed by the crush-dressing roller 28 so that they are equal in diameter and have crest surfaces 500: of equal width. As before, the depth of ridges 48a is made less than one half the depth of the desired thread. The root surfaces 52a are all of equal width and are of a diameter which is slightly smaller than that of the adjacent cylindrical grinding surface 4401-. Subsequent to the crush-dressing operation, the crests Eta of all except the last ridge 4612 are trimmed or: on a bevel by means of a diamond dressing tool such as is also used to form the plain surfaces and Me. As a consequence, the final crest surfaces 5 5a gradually increase in width toward the work entrance side 48a of the wheel while the depth of ridges 45a gradually decreases in the same direction.

The grinding wheel shown in Figure 3 is preferred for use on all hard materials. The narrow crests of the wheel shown in Figure 2 will seize a work piece more quickly and cause it to move faster axially than the wider crests of Figure 3 which initially have more metal to remove from the surface of the work blank. However, on hard materials the ridges of the Wheel of Figure 2 wear down rather rapidly and therefore are preferred for use in grinding softer metals such as powdered metals.

Another modification of the preliminary grindwheel. As before, the final crest surface 56b protrudes above the cylindrical grinding surface Mb by an amount les than one half the depth of the desired thread. This modification has the same operating characteristics as that of Figure 2, the portions of ridges 46 and 451) which come into contact with the work being the same.

Thus it will be seen that the above stated objects of the invention have been attained by the provision of centerless thread grinding machines, grinding wheels and methods which increase the rate of production, decrease wear on the threads of the grinding wheel and increase the accuracy of the finished product. These commercially important results are achieved by the provision of novel means and methods for initially seizing the work blank and initiating the grinding process more quickly while at the same time establishing and maintaining an exact correlation between the rotative speed of the work blank and its axial rate of advance. In this connection it is to be particularly noted that although the various forms of the invention may establish different rates of axial advance of the work piece, the rates depending largely upon the amount of metal removed by the initial rough grinding thread ridges, in each case the rate of axial advance is properly and necessarily precisely correlated to the rotative speed of the work blank so that the blank is delivered to the first roughing thread generator ridges at the proper rate of speed.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by United States Letters Patent is:

1. In a grinding wheel for grinding threads on a work piece having successively arranged grinding zones including a thread finishing zone, a rough grinding zone and a cylindrical zone for grinding the external diameter of said work piece, a control section intermediate the entrance side of said wheel and said cylindrical zone, said control section having a plurality of ridges for forming incipient threads to establish a predetermined axial movement of said work piece before it reaches said cylindrical zone.

2. A grinding wheel according to claim 1 wherein the diameter of the root surfaces of the incipient thread grinding ridges is less than the diameter of said cylindrical grinding zone.

3. A grinding wheel according to claim 1 wherein the length of said cylindrical grinding zone is such that the work piece is engaged by the rough grinding zone before it leaves the control section.

4. In a method of grinding threads in a work piece, the steps of grinding incipient threads on said work piece, grinding the external diameter of said work piece, and initiating a rough grinding operation in certain of said incipient threads prior to completion of the grinding or others of said incipient threads.

CHARLES A. REIMSCHISSEL.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,098,145 Harley Nov. 2, 1937 2,420,504 Stewart May 13, 1947 2,501,389 Hopkins Mar. 21, 1950 FOREIGN PATENTS Number Country Date 633,371 Great Britain Dec. 12, 1949 

